Selective catalytic reduction (SCR) of nitrogen oxides can be carried out using a zeolite promoter with a reductant such as ammonia, urea, and/or hydrocarbon in the presence of oxygen. While a variety of zeolite forms are known, certain forms, such as zeolite beta and chabazite have been particularly utilized for forming metal-promoted catalysts, particularly for SCR applications.
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, about 200° C. to about 600° C. or higher, under hydrothermal conditions, which are often encountered such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Metal-promoted zeolite catalysts useful in SCR have included, among others, iron-promoted zeolite catalysts and copper-promoted zeolite catalysts. For example, Iron-promoted zeolite beta has been described in U.S. Pat. No. 4,961,917. The process of preparation of metal containing Chabazite particularly can include exchange of the desired metal species with accompanying removal of alkali metals, such as sodium, which can be detrimental to the hydrothermal stability of the final catalyst. The typical Na2O level of Na-Chabazite is between 6,000 and 8,000 ppm. Sodium is known to degrade the zeolite structure under hydrothermal aging conditions via formation of Na4SiO4 and Na2Al2O4 and concomitant dealumination of the zeolite.
Previous attempts to form Iron-exchanged zeolite using the chabazite form, have proven difficult. For example, U.S. Pat. Pub. No. 2015/0231620 proposes a method of preparing Iron-zeolite chabazite without ion exchange since the publication points out that it has been difficult to incorporate iron into chabazite zeolites such as SSZ-13 using traditional ion-exchange methods due to the small pore openings of the chabazite structure (e.g., in the 3-4 Angstrom range). The publication posits that its direct incorporation of iron during synthesis of the chabazite is preferred because incorporating iron into chabazite with ion exchange is not feasible due to the small pore size of chabazite. Nevertheless, because of the prevalence of utilization of ion-exchange in forming promoted zeolite catalysts, it would be useful to provide further methods for formation of Iron-exchanged zeolites, particularly Iron-exchanged chabazites and other zeolites with characteristics similar to chabazite forms.